Why comets grow tails — Astronomy, 7–10 years
A comet can change from a dark, frozen lump into a bright object with a long tail when it comes near the Sun.
A comet is a traveller made of ice and dust
Far from the Sun, a comet is a cold, dark ball of frozen gases mixed with dust and rock. Near the Sun, its ice warms and turns into gas, carrying dust away and making a glowing cloud and tail.
Why does the tail appear?
People once saw comets suddenly appear and disappear, so they searched for a pattern. The problem was explaining how one small object could become enormous in the sky. Sunlight and a stream of particles from the Sun push the released gas and dust away from the Sun.
Halley’s comet, step by step
1. Halley’s comet was seen from Earth in 1986. 2. It travels around the Sun and returns about every 76 years. 3. Its next return is expected in 2061. 4. Near the Sun, warming ice releases gas and dust, so sunlight helps make its bright tail visible.
The tail does not trail behind
It is natural to think a comet’s tail streams behind it like a flag on a moving bicycle. In fact, sunlight and solar particles push the tail generally away from the Sun, even when the comet is moving in another direction. That is why its direction can change along its path.
Comets as time capsules
Comets formed from material left over when the Solar System began, so they can preserve clues about that distant time. Space missions have flown close to comets, photographed their surfaces and studied their dust. This helps scientists test ideas about how planets formed.
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